The basic structure of an organic light emitting diode (OLED) includes an anode layer, functional layers and a cathode layer. The functional layers include a hole transport layer, a light emitting layer and an electron transport layer. When a proper voltage is provided across the cathode and the anode, electrons and holes are injected from the cathode and the anode into the electron layer and the hole transport layer, and further migrate to the light emitting layer via the electron layer and the hole transport layer, respectively. The holes and electrons combine within the light emitting layer to generate light, thereby achieving the property of light emitting of the OLED device by itself.
In order to increase the light emitting efficiency, the anode material for the OLED device must have a high work function and light transmittance, while the cathode material generally requires a low work function. Therefore, indium tin oxide (ITO) transparent conductive film, which has a work function of 4.5 eV˜5.3 eV and a stable property and allows light to transmit, is widely used as the anode material for an OLED. A metal having a low work function, such as Al, Ca, Li, Mg, etc., or a composite metal with a low work function is generally used as the cathode material. However, the work function of an ITO film or various metals itself is constant, and increasing the work function of the ITO film or decreasing the work functions of metals is primarily achieved by conducting a surface treatment on them. Therefore, there is a limited range within which the work function of electrodes of the prior art OLED devices can be adjusted, which limits the increase of the display efficiency of an OLED device.
Moreover, active matrix organic light emitting diode (AMOLED) display device essentially consists of thin film transistors (TFTs) and OLEDs. A TFT comprises structures such as a gate electrode, a gate insulation layer, a source layer, drain and source electrodes, etc. An OLED comprises structures such as an anode layer, a hole transport layer, a light emitting layer, an electron transport layer, a cathode layer, etc. The anode or cathode of the OLED is connected to the TFT. This complicated structure causes an issue that an AMOLED has to be prepared in a complicated manufacture process and high production costs.